1. Field of the Invention
The present invention relates to a communication circuit preferably for use in an RFID (Radio Frequency Identification) system or a near field communication (NFC: Near Field Communication) system, in which communication with a target device is performed by an electromagnetic field signal.
2. Description of the Related Art
NFC is one of the near field communication standards using a frequency band of 13 MHz, and expected to be applied to various terminals including mobile communication terminals. A mobile communication terminal using NFC typically has an RFIC for NFC built in a main body of the terminal, and the RFIC for NFC is connected to an antenna coil for NFC that is also built within the terminal main body. Further, the antenna coil is connected to a capacitance element so as to resonate at a communication frequency, and the capacitance element and the antenna coil constitute an antenna circuit. In addition, the antenna circuit and the RFIC for NFC or the like constitute a wireless communication module (hereinafter referred to as “NFC module”).
While a communication frequency for the NFC module is previously determined, a resonant frequency to which the antenna circuit is to be tuned varies to some degree depending on its use conditions and a production tolerance. For example, circuit architecture of the antenna circuit as a resonance circuit is different between a reader/writer mode and a card mode. Accordingly, it is necessary to adjust the resonance circuit according to the mode so that a predetermined resonant frequency is maintained in both modes. Further, the use conditions change according to an environment in which the NFC module is installed. For example, the resonant frequency of the antenna circuit changes depending on whether or not there is metal near the NFC module.
If a frequency band of the antenna in the NFC module is sufficiently broad, fine adjustment due to the difference in the use conditions is not necessary. However, it has become difficult to ensure an adequate antenna size as recent terminals are increasingly downsized, and the antenna's bandwidth may not be broadened if the size of the antenna is small. Therefore, it is necessary to adjust the resonant frequency to obtain an optimal value.
As one method of adjusting the resonant frequency, there is known an antenna circuit including a capacitor configured by a variable capacitance element capable of changing a capacitance value by an applied voltage (see, for example, Japanese Patent Unexamined Publication No. 2009-290644). Alternatively, Japanese Patent Unexamined Publication No. 2010-147743 discloses a circuit that switches between entire capacitance values by selectively connecting a plurality of capacitors.
FIG. 10 is an example of a communication circuit disclosed in Japanese Patent Unexamined Publication No. 2010-147743. In the drawing, a non-contact IC unit 47 is configured by a non-contact IC chip, an antenna parallel capacitor unit having a capacitor Cin, parallel capacitors C1 to C3, and the switches SW1 to SW3, and an antenna L1. Values of electric capacitances of the capacitor Cin and the parallel capacitors C1 to C3 are static. The switches SW1 to SW3 are circuits for switching between ON and OFF of the parallel capacitors C1 to C3, respectively. After the non-contact IC unit 47 is incorporated in a mobile telephone 1, a control IC 62 having a non-volatile memory built in is connected to the non-contact IC unit 47. The control IC 62 controls the switches SW1 to SW3 of the non-contact IC unit 47 to switch between ON and OFF of the switches SW1 to SW3.
However, when a variable-capacitance diode and a switching circuit are provided, it is necessary to provide a space for mounting these active elements, and there is often a case in which the resonant frequency changes because distortion may easily occur since these elements are active elements. Further, terminals for receiving and transmitting signals and data for adjusting the resonant frequency of the antenna circuit, as well as lines for transmitting these signals and data, are required. In addition, a large number of capacitors and switches are necessary in order to adjust the capacitance value in fine steps by switching between the plurality of capacitors. This adversely complicates the circuit architecture, and increases the size of an IC.